This invention relates to spherical resin particles in micron order with a narrow particle diameter distribution, to methods for the production thereof and to uses thereof. Specifically, this invention relates to resin particles which have been widely used as matting agents, blocking-preventive materials, carriers for chromatography, carriers for medicaments, powder paints and varnishes, gap-adjusting materials, toners for electrophotography, cosmetics, and the like.
In recent years, polymer beads have been put into wide use as matting agents, blocking-preventive materials, organic pigments, carriers for chromatography, carriers for medicaments, powder paints and varnishes, gap-adjusting materials, toners, and the like. Polymer beads used for such purposes are required to have a particle diameter ranging from 0.1 to 100 .mu.m, a narrow particle diameter distribution and a spherical shape.
As examples of the resin particles hitherto usable for the above-mentioned purposes, mention can be made of resin particles which are prepared by a polymerization granulation method. Polymerization granulation methods are generally classified into emulsion polymerization method, suspension polymerization method, seed polymerization method and dispersion polymerization method, which are discussed hereunder.
Emulsion, polymerization method
In this method, resin particles are obtained by polymerization in the micelles of polymerizable monomers stabilized by surfactants in water.
According to the emulsion polymerization method, particles having a sharp particle diameter distribution can be obtained. However, the particle diameters are decided by the size of the micelles existing stably, confining the diameter range to from about 0.01 to about 0.5 .mu.m, and it is not possible to prepare particles having a particle diameter of about 1 .mu.m or more. Besides, surfactants essential for stabilization of the micelles remain on the surface of the particles prepared, which also limits the applicable use of the obtained resin particles.
Suspension polymerization method
According to this method, polymerizable monomers are polymerized to afford particles in a suspension system obtainable by mechanically stirring a mixture of water and the polymerizable monomers.
In the suspension polymerization method, it is not easy to polymerize in a stable system. In addition, it is difficult to obtain fine polymer particles having a uniform particle diameter distribution, since the particle sizes depend on the mechanical stirring.
For this reason, a suspension-stabilizing agent is used in suspension polymerization to prevent particles from coalescence and to stabilize the polymerization. As such suspension-stabilizing agents, use is generally made of sparingly soluble inorganic compounds, for example, sparingly soluble salts such as barium sulfate, calcium sulfate, magnesium carbonate, barium carbonate, calcium carbonate and calcium phosphate; metal oxides such as silica, calcia, magnesia, titanium oxide; minerals such as diatomaceous earth, talc, clay and kaolin; and their mixtures or water-soluble mixtures, such as polyvinyl alcohol, gelatin and starches.
Actually, even when said suspension-stabilizing agents are used, the particle diameters of the particles obtained by the suspension polymerization method are about several dozens .mu.m or more and the particle diameter distribution thereof is broad, which gives rise to the classification after polymerization.
Seed polymerization method
The seed polymerization method has been proposed to solve the above-mentioned problems. Therein, particles obtained by another method are used as seed particles, and are imbibed with solvents and a polymerizable monomer. The thus-obtained particles are allowed to grow by polymerization within the imbibed seed particles.
In the seed polymerization method, it is, in principle, possible to obtain particles having a sharp particle diameter distribution by selecting suitable seed particles, and the particle diameters can be controlled according to the imbibition ratios of the seed particles and polymerizable monomers.
The seed polymerization method was originally devised for the purpose of producing particles having a particle diameter between 0.01-0.5 .mu.m obtainable by the emulsion polymerization method and several dozens .mu.m or more obtainable by the swelling polymerization method. Accordingly, as a matter of fact, the particles usable as seed particles in the industrial seed polymerization are often limited to particles obtainable by the emulsion polymerization method, namely, vinyl polymer particles. However, it is difficult to imbibe vinyl polymer particles using polymerizable monomers. The imbibition ratio is decided by the interaction of the polymer composing the seed particles with the monomer used for imbibition and by a balance with a surface tension of the imbibed particles, and thus the imbibition ratio is actually limited to 2-10 times at most.
Alternatively, the imbibition ratio cannot be increased extremely, since the particle diameters which can be enlarged at one time is limited. Ten times the particle diameter corresponds to 1,000 folds the volume, and extreme imbibition necessitates repeated seed polymerization.
Two-step imbibition seed polymerization method
This method was devised for the purpose of increasing the imbibition ratio of seed particles. In the 2-step imbibition seed polymerization method, seed particles are imbibed with an oligomer or a sparingly soluble lower molecular substance (a imbibing agent), etc., whereafter the particles are further imbibed with a polymerizable monomer. By this method, the imbibition ratio of the seed particles can be increased to several thousand folds. However, the imbibing agent remains in the particles obtained by the 2-step imbibition seed polymerization method, and another step for removing this imbibing agent is requisite.
Though the seed polymerization method is excellent in that the resin particles in micron order having a sharp particle diameter distribution can be prepared, the foregoing problems prevent the seed polymerization method from being industrially employed.
Dispersion polymerization method
In this method, a polymerizable monomer, an initiator and a stabilizer are dissolved in an organic solvent, whereby initiating the polymerization, and the polymer particles insoluble in the organic solvent are made to grow with coalescence of the oligomers produced in the first stage of the polymerization as the particle cores.
Though the dispersion polymerization method is excellent for preparing resin particles in micron order having a sharp particle diameter distribution, realization of mass-production by this method is difficult, due to the use of an organic solvent as a medium, which disqualifies the dispersion polymerization method as an industrial method for producing spherical resin particles.
As mentioned above, the particle diameter range of resin particles is limited in the emulsion polymerization method and the suspension polymerization method, and besides, only resin particles having a broad particle diameter distribution can be obtained by these methods. It is possible to obtain resin particles having a sharp particle diameter distribution by the seed polymerization method and the dispersion polymerization method. However, it is in effect impossible to produce spherical resin particles at a low cost, since realization of mass-production by these methods is unavailable.
Moreover, as mentioned above, resin particles produced by the polymerization-granulation methods, i.e., emulsion polymerization, suspension polymerization, seed polymerization and dispersion polymerization, are in most cases limited to vinyl polymers, as is self-evident from the production steps therefor. Thus, the resin particles of the condensation polymers of the present invention cannot be obtained by the "polymerization-granulation" method.
On the other hand, particles of polyester resins, polyamide resins, polyurethane resins and other condensation type resins cannot be formed by a suspension polymerization means. As an alternative means, resins are dissolved in a solvent and this solution is added dropwise to a precipitating agent for solidification, but it is difficult to form substantially spherical particles by this way, and a step for recovering the solvent is required.